High-capacity computer storage devices typically include one or more electromagnetic transducers and a corresponding number of magnetic media disks. The transducers, also known in the art as "heads", are adapted for transfer of electronic information between a data source, for example a computer, and data locations on the magnetic disks. Information is communicated in accordance with well-known conventions and formats that enable high-density storage, rapid access to data locations, high reliability, data integrity, and device miniaturization, writing and reading of data is accomplished with a read/write head positioned over a disk.
In general, information is stored on and read from disks in "tracks", which are regions of width T which extend along a plurality of concentric circular track center lines disposed about a disk axis. There are inter-track regions which are not used for storage of data. In some prior art disks, relatively high frequency "servo burst" signals are written in the intertrack regions, for use in maintaining a read/write head on or near the track center line, so that optimal reading and writing can be effected.
In the manufacture of a disk-based storage system, known as a "disk-drive", magnetic heads are usually tested on special magnetic head/disk testers with regard to their parameters and performance characteristics. An example of a prior-art head/disk tester (U.S. Pat. No. 5,382,887) is shown in FIG. 1, a schematic top view of the tester. In that figure, the tester has a carriage 10 that is shown in reference to a horizontal X-Y coordinate system affixed thereto. Carriage 10 is slidable along horizontal bottom rails 16 and 18 mounted on a base 23 and extending along the X axis. Carriage 10 houses an outer ring 15 affixed thereto that supports an inner ring 13. Inner ring 13 is rotatable about a vertical (Z) axis extending from the centrally located (with respect to ring 15) intersection of the X and Y axes. Ring 13 carries a magnetic head support 12 with a magnetic head 14 to be tested.
The magnetic head 14 is located in the center of the rings 13 and 15. A magnetic disk support (not shown) extends vertically from base 23 and provides support for a magnetic disk 11 in a horizontal plane, with disk 11 being rotatable about a vertical spin axis. As carriage 10 slides along rails 16 and 18, in the direction of the X axis, the magnetic head 14 moves across the magnetic disk 11 in the directions indicated by arrow A. A stepper motor (not shown) selectively rotates inner ring 13 (about the Z axis) with respect to outer ring 15 and carriage 10. A stepper motor 19 affixed to base 23, rotationally drives a lead screw 21 which is coupled to an intermediate block 21A which is slidable in the X direction along rails 21B and 21C. The intermediate block 21A is coupled to carriage 10 by a piezo actuator 17. With this configuration, the position of carriage 10 (and thus head 14) along the X axis is controlled in a course manner by motor 19, lead screw 21 and block 21A. The relative position of carriage 10 with respect to block 21A about the axis is controlled in a fine manner by piezo actuator 17.
Linear encoders 20 and 22 are disposed in opposite sides of carriage 10, parallel to the direction of the carriage movement and symmetrical with respect to the centerline of the carriage 10. Each encoder consists of two portions: a stationary portion, namely, parts 20a and 22a that are attached to base 23, and a movable portion, namely, parts 20b and 22b, that are attached to outer ring 15. In one form, the current position of the head 14 is defined by arithmetic mean of readouts of the encoders 20 and 22.
The difference between the desired position and a current position of the head 14 is determined by a controller-amplifier 25, which provides a feedback control signal that is applied to the piezo actuator 17. In response to that signal, the piezo actuator 17 adjusts its dimension in the X direction. As a result, the carriage 10 moves in order to locate the head 14 to the desired position.
When the room temperature changes during testing, all parts of the head/disk tester, in particular, the carriage 10, the magnetic head holder 12, and the magnetic head 14, change their dimensions due to thermal expansion or contraction. These dimensional changes are not the same for different parts of the head/disk tester because of the differences in thermal expansion coefficients of the various parts. Moreover, the rate of temperature change is not the same for different parts due to differences in their heat transmission and storage characteristics. As a result, arithmetic mean of readouts of the encoders 20 and 22 does not reflect the true location of the head 14 under conditions of changing temperature.
Even when the room temperature is constant, different parts of the head/disk tester heat up differently depending on their respective proximities to heat sources in the tester (e.g., motors). Further, the dimensions of the magnetic disk 11 also change when the temperature changes. As a result, the location of the magnetic head 14 with respect to the magnetic disk 11 is not predictable in the case of a temperature change.